Could be they used powders in that burn rate to keep pressure down while still getting the advertised velocity.  I pulled down some 30 Remington that spanned that era, the earlier ones had a stick powder and the charges would have gone along with 3031, which the powder also resembled in grain size.  Later the rounds were loaded with a ball powder and the case was full with a uniform ball powder that looked like 760.  Good low pressure loads for some very old guns, like my Remington 14.  Still later and I can see that the balls weren't the same uniform size as they were in the previous loads.  Perhaps a blended powder like Hodgdon's Superformance or LVR.